High speed tractor drawn earthworking scrapers include a bowl which has sidewalls and a floor with a forward cutting edge, and an apron which is pivoted near the top of the scraper for movement between an elevated position which it occupies when the scraper is being dumped, an intermediate position somewhat above and forward of the cutting edge in which it assists with bowl loading, and a closed position against the bowl floor adjacent the cutting edge to retain a load in the bowl during transport.
Ever since the introduction of such scrapers, there have been problems with the operation of the aprons. The simplest type of apron is a one-piece plate, usually curved, having mounting arms that are pivoted on the bowl sidewalls so that the lower edge of the apron moves from elevated position to closed position in an arc of a circle struck about the axis formed by the mounting pivot. The path of approach of the lower edge of the apron during final closing is such that it is very easy for a rock to be caught between the lower edge of the apron and the bowl floor, thus preventing the apron from closing completely; and it is also not uncommon for the apron to be jammed out of contact with the bowl floor by reason of a solid mass of dirt piled on the ground below it.
U.S. Pat. No. 2,344,313 discloses one early proposal for solving the foregoing problem of apron hangup.
U.S. Pat. Nos. 3,471,952 and 3,574,960, both owned by applicant's assignee, disclose multiple articulated apron mountings the purpose of which is to alleviate the apron closing problems.
U.S. Pat. Nos. 3,739,506, also owned by applicant's assignee, discloses a two-piece scraper apron which was also developed to minimize the functional problems related to such aprons.
In general, the systems of the prior art patents are either functionally unreliable or excessively complex and expensive, or both.